TRS-80 (IV) help?

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Mon Oct 11 23:47:58 2004

>
> Hi Guys,
>
> Picked up another big load this weekend, and am just starting to go
> through it - Item#1 is a TRS-80 Model IV.
>
> When I first powered it up, it sucked on the Empty bottom drive for
> a couple of seconds, then prompted with "Cass?" and entered ROM basic.

This means it ccouldn't find a disk controller. The Cass? prompt is to
select High (1500 buaf) or Low (500 baud) cassette operation. The next
prompt is 'Memory Size' (to specify the highest address that will be used
by BASIC, thus allowing you to reserve memory for machine code programs,
etc), then you go into BASIC.

> Tried a Model IV boot disk and go the same result - so I removed the
> drive and cleaned it (machine has been sitting a long time).
>
> Now, it powers up, selects the drive and "hangs" - does not move on
> to the "Cass?" prompt - I've checked all connections and socketed
> chips, and everything is clean and looks OK. Have reseated everthing
> several times.

Have you checked -- I mean _really_ checked -- the ribbon cable between
the CPU board and the disk controller board. That's a common cause of
this sort of problem.

> - If I disconnect the disk controller board ribbon cable, the machine
> powers up at the "Cass?" prompt - in this case, I believe the ROM
> startup sees that there is no disk system and enters ROM basic,
> thinking that it is a diskless unit.


This is correct, and it's what you should see.

>
> - If I power-up/reset while holding BREAK, it also gives the "Cass?"
> prompt - after briefly selecting the drive - looks like BREAK very
> soon after power-up can interrupt the boot process, and ROM BASIC
> comes up normally.

Yes. Resetting with BREAK held down will get the machine to ignore the
disk system and pretend to be a cassette-only machine.

>
> - I never see the drive seek - it selects, but does not seek.
> If I manually move the drive head out during power-off, it DOES NOT
> seek back to track-0 after selecting the drive when I power it on.

> Can anyone tell me what the Model IV is *SUPPOSED* to do on power-up, both
> with and without a boot disk in the drive (I have no documentation at all).
>

Roughly what you're seeing...

My guess is that it's not talking properly to the 1793 disk controller
chip. I have had this chip fail :-(. But you might start by checking the
power supply (there's a second PSU board in disk units, mounted on the
side of the drive tower, which supplies the disk controller board and
drives). And then check and re-check that ribbon cable.

> Any pointers to service documentation? ROM listings? Other information on
> servicing a model IV - any idea how I can further diagnose this problem?

I have schematics. FWIW, the disk controller board is the same as in the
Model 3, should you be able to find info on that machine.

-tony
Received on Mon Oct 11 2004 - 23:47:58 BST